Concerning a gasoline engine, hazardous components in exhaust gas has been steadily decreased by strict regulations of exhaust gas and advance in technology that can deal with the regulations. However, concerning a diesel engine, there are still many technical problems left with purification of exhaust gas because the exhaust gas contains particulates (particulate matter: such as soot composed of carbon particulates or soluble organic fractions (SOF), hereinafter referred to as PM).
Accordingly, in recent years, the oxidation catalyst capable of oxidizing and removing PMs, particularly soot components, at low temperature have been developed. For example, Japanese Unexamined Patent Application Publication No. 2004-42021 (Document 1) discloses a catalyst composition containing silver (Ag) and/or cobalt (Co) stabilized ceria (CeO2), which facilitates soot oxidation during the regeneration of DPF. Here, a method of manufacturing the catalyst described in Document 1 is a method of obtaining the catalyst composition having a porosity wherein a pore size is centered around 70 to 200 Å and a high specific surface area ranging from 14 to 150 m2/g by impregnating a cellulose material (Whatman (registered trademark) Filter Paper 540) with a nitrate precursor and burning the cellulose at 600° C. and for two hours after drying the material overnight at a room temperature. However, it was not possible to oxidize carbon-containing components, such as soot, and components, such as HC, CO or NO, sufficiently at low temperature by the catalyst compositions disclosed in Document 1, i.e. merely by presence of a set of Ag and CeO2 or a set of Ag, Co and CeO2.
Meanwhile, in recent years, among PMs emitted from diesel engines, PM2.5 which has a particle diameter of 2.5 μm or less has become a problem. Among others, development of technology for removing PMs having particularly smaller particle diameters has been requested. For example, Japanese Unexamined Patent Application Publication No. 2006-7148 (Document 2), discloses a filter for purification of exhaust gas in which multiple filters are arranged along a direction of a flow of exhaust gas. The filter for purification of exhaust gas has a wall-through-type filter and a filter having a needle-like material or fibers formed on a wall thereof which is in contact with exhaust gas. However, there have been problems that one, as described in Document 2, having two kinds of filters in combination has a complex configuration, and that a needle-like material comprising an inorganic material which is desirably avoided for use if possible.